In general, a liquid crystal display (LCD) refers to a device that displays numerals or images through application of an electric field to liquid crystals, which are a material having an intermediate phase between a liquid and a solid, and arranged between two glass substrates acting as electrodes.
Since the LCD is not an emissive device, it must be provided with a backlight unit as a light source for emitting light. Then, images and the like are displayed on a liquid crystal panel having liquid crystals arranged in a predetermined pattern by adjusting a transmittance amount of light emitted from the backlight unit.
FIG. 1 is an exploded perspective view of a conventional LCD backlight unit.
According to a position of a light source for emitting light, a backlight unit 10 of the liquid crystal display can be classified into a direct type backlight unit in which the light source is located directly at the rear of an LCD panel 100, and an edge type backlight unit in which the light source is located at a lateral side of the LCD panel 100. FIG. 1 shows an edge type backlight unit 10.
Referring to FIG. 1, the conventional LCD backlight unit includes a light source 105, a light guide panel 110, a reflection plate 115, a diffusion sheet 120, prism sheets 125, and a protector sheet 130.
The light source 105 serves to emanate light initially to the liquid crystal display. Although various types of light source can be used, the LCD generally employs a cold cathode fluorescence lamp (CCFL) that requires very low power consumption and enables emission of very bright white light.
The light guide panel 110 is located at one side of the light source 105 below the LCD panel 100, and serves to project light in front of the light guide panel after converting spot light of the light source 105 to plane light.
The reflection plate 115 is located at the rear of the light guide panel 110, and serves to reflect light emitted from the light source 105 towards the LCD panel 100 in front of the reflection plate.
The diffusion sheet 120 is positioned at an upper side of the light guide panel 110, and serves to uniformize light having passed through the light guide panel 110.
The prism sheets 125 serve to improve brightness by refracting and collecting light that tends to undergo a rapid decrease in brightness due to diffusion in both directions, i.e., horizontal and vertical directions while passing through the diffusion sheet 120.
The protector sheet 130 is disposed on the prism sheet, and serves to protect the prism sheets 125 from damage such as scratches and to prevent the Moire phenomenon that occurs when using the prism sheets 125 stacked in two layers in the horizontal and vertical directions.
Although not shown in FIG. 1, the backlight unit 10 further includes a mold frame or housing, which serves to secure respective components of the backlight unit 10 so as to allow the backlight unit 10 to be prepared as an integral component, and a back cover or lamp cover, which serves to protect and support the backlight unit 10 while maintaining the strength of the backlight unit 10.
The LCD can be classified into a twisted nematic (TN) type and an in-plane switching (IPS) type depending on the arrangement of liquid crystals. The TN type LCD has an inferior viewing angle to the IPS type LCD. However, since it has excellent transmittance, the TN type LCD is suitable for an LCD that requires front visibility. On the other hand, although the IPS type LCD has an excellent viewing angle compared to the TN type LCD, it has lower transmittance, which deteriorates overall brightness.
Thus, there is a need for an enhancement in brightness of LCD at a specific angle or compensation for brightness at a specific position depending on operation surroundings, kinds of LCD panel related to liquid crystals arranging manners, and other external factors.
Conventionally, a dual brightness enhancement film (DBEF) and a diffusive reflective polarization film (DRPF) have been used to compensate for brightness or to improve a viewing angle. However, these films lead to an increase in total thickness of a backlight unit along with an increase in manufacturing cost, thereby lowering competitiveness of LCD products adopting such films.
Item of brightness uniformity-viewing angle dependency in TCO'03 standard regulates that degrees of brightness uniformity with respect to horizontal and vertical views of a general visual display unit should be managed to fall within a specified range.
FIG. 2 shows measurement positions defined in the TCO'03 standard for characteristics evaluation of a general visual display unit. FIGS. 2 and 3 show the TCO'03 standard with respect to brightness uniformity-viewing angle dependency.
In the TCO'03 standard, degrees of brightness uniformity with respect to horizontal and vertical views are respectively defined as LH and Lv, and are regulated to have 1.7 or less. The TCO'03 standard is one of severe obstacles in development of TN mode liquid crystal displays (LCDs) having a relatively large restriction in viewing angle of a display. Accordingly, there is a need of components or techniques for controlling a viewing angle at a specific angle and brightness at a specific position in an LCD backlight unit.